Optical pickup capable of preventing tilting variation of an optical axis of an objective lens during movement thereof

ABSTRACT

In an optical pickup ( 1 ) in which an objective lens ( 6 ) is moved in a focusing direction by the use of a focusing coil ( 7 ) and first and second magnetic circuits ( 8,9 ) coupled to the focusing coil, the first and the second magnetic circuits have a first and a second magnetic flux density, respectively. The objective lens is held by a lens holder ( 5 ) which is elastically coupled to a damper base ( 2 ) through a suspension member ( 3,4 ) to be movable in the focusing direction. The lens holder has a first part and a second part which are spaced from each other. In cooperation with the focusing coil, the first and the second magnetic circuit generate first and second force urging a first and a second part of the lens holder in the focusing direction, respectively.

BACKGROUND OF THE INVENTION

This invention relates to an optical pickup for use in an opticalapparatus such as an optical disk drive and, more particularly, to anoptical pickup capable of preventing an inclination of an optical axisof an objective lens upon focusing an optical beam on an optical disk.

Referring to FIGS. 1 and 2, a conventional optical pickup 1 comprises adamper base 2, two pairs of suspension wires 3 and 4 (in the figures,one suspension wire alone is illustrated in each pair) having one endsfixed to the damper base 2, a lens holder 5 attached to the other endsof the suspension wires 3 and 4 and elastically supported by thesuspension wires 3 and 4 to be swingable and returnable to apredetermined position, an objective lens 6 held by the lens holder 5, afocusing coil 7 attached to the lens holder 5, a first magnetic circuit8 intersecting one side of the focusing coil 7 which is faced to thedamper base 2, and a second magnetic circuit 9 intersecting the otherside of the focusing coil 7 which is opposite to the one side.

In the conventional optical pickup 1, the first magnetic circuit 8 has amagnetic flux density equal to that of the second magnetic circuit 9.Therefore, a first force F1 produced between the first magnetic circuit8 and the focusing coil 7 has an intensity equal to a second force F2produced between the second magnetic circuit 9 and the focusing coil 7.

The optical pickup 1 further comprises a tracking coil, an optical base,a laser diode, a polarization beam splitter, a collimator lens, areflection mirror, and a photo diode, although not shown in the figure.These components are irrelevant to the gist of this invention and willnot be described herein.

The conventional optical pickup 1 is designed so that an optical axis Oof the objective lens 6 is kept in parallel to the vertical directionwhen the lens holder 5 is moved in a focusing direction (tangentialdirection) FC of the objective lens 6. In practical use, however, due tovarious factors known in the art, movement of the lens holder 5 in thefocusing direction FC of the objective lens 6 causes predeterminedtilting variation of the optical axis O of the objective lens 6 ineither a first mode or a second mode which will presently be described.

In the first mode, when the lens holder 5 is shifted upward from thepredetermined position, the optical axis O, particularly, an upperportion thereof is inclined towards the damper base 2 as shown in FIG.2A. When the lens holder 5 is shifted downward from the predeterminedposition, the upper portion of the optical axis O is inclined away fromthe damper base 2 as shown in FIG. 2B.

In the second mode, when the lens holder 5 is shifted upward from thepredetermined position, the upper portion of the optical axis O isinclined away from the damper base 2 as shown in FIG. 3A. When the lensholder 5 is shifted downward from the predetermined position, the upperportion of the optical axis O is inclined towards the damper base 2 asshown in FIG. 3B.

Upon occurrence of the tilting variation of the optical axis of theobjective lens, the optical pickup is deteriorated in characteristics.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an optical pickupwhich is capable of preventing tilting variation of an optical axis ofan objective lens during movement of a lens holder in a focusingdirection of the objective lens to thereby prevent deterioration incharacteristics of the optical pickup.

Other objects of the present invention will become clear as thedescription proceeds.

According to the present invention, there is provided an optical pickupwhich comprises a damper base, a lens holder having a first part and asecond part which are spaced from each other in a predetermineddirection, a suspension member elastically coupling the lens holder tothe damper base to make the lens holder be movable relative to thedamper base in a focusing direction perpendicular to the predetermineddirection, an objective lens held by the lens holder, a focusing coilfixed to the lens holder, a first magnetic circuit connected to thedamper base and coupled to the focusing coil for generating first forcein cooperation with the focusing coil to urge the first part of the lensholder in the focusing direction, the first magnetic circuit having afirst magnetic flux density, and a second magnetic circuit connected tothe damper base and coupled to the focusing coil for generating secondforce in cooperation with the focusing coil to urge the second part ofthe lens holder in the focusing direction, the second magnetic circuithaving a second magnetic flux density different from the first magneticflux density.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a characteristic part of a conventionaloptical pickup;

FIGS. 2A and 2B are for describing a first mode of tilting variation ofan optical axis in the conventional optical pickup illustrated in FIG.1; and

FIGS. 3A and 3B are for describing a second mode of tilting variation ofan optical axis in the conventional optical pickup illustrated in FIG.1; and

FIG. 4 is a sectional view of an optical pickup according to anembodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Now, description will be made of this invention with reference to thedrawing.

Referring to FIG. 4, an optical pickup 1 according to an embodiment ofthis invention comprises a damper base 2, two pairs of suspension wires3 and 4 (in the figures, one suspension wire alone is illustrated ineach of the upper and the lower pairs 3 and 4), a lens holder 5, anobjective lens 6, a focusing coil 7, a first magnetic circuit 8, and asecond magnetic circuit 9. The optical pickup 1 further comprises atracking coil (not shown), an optical base 11, a laser diode 12, apolarization beam splitter 13, a collimator lens 14, a reflection mirror15, and a photo diode (not shown).

The optical base 11 has a rear wall 111 and both side walls 112. On theoptical base 11, the laser diode 12, the polarization beam splitter 13,the collimator lens 14, the reflection mirror 15, and the photo diodeare mounted.

The damper base 2 has a generally rectangular parallelepiped shape andis provided with holes (not shown) for receiving one ends of thesuspension wires 3 and 4 to hold the suspension wires 3 and 4. Thedamper base 2 is rotatably attached to the rear wall 111 of the opticalbase 11. The one ends of the suspension wires 3 and 4 are inserted intothe holes of the damper base 2 and fixed to inner walls of the holes bythe use of an adhesive (not shown).

The lens holder 5 has a lens holding portion 51 and a bobbin portion 52.The lens holding portion 51 has a generally cylindrical shape. Thebobbin portion 52 has a generally cylindrical shape and surrounds thelens holding portion 51 to be coaxial with the lens holding portion 51.The lens holding portion 51 and the bobbin portion 52 are connected toeach other by a connecting member (not shown) to form a gap 53therebetween. The bobbin portion 52 includes a first part and a secondpart which are opposite to each other in a predetermined direction alonga sheet plane of FIG. 4. In other words, the first and the second partsare spaced from each other in a radial direction of the bobbin portion52.

The other ends of the suspension wires 3 and 4 are inserted into thefirst part of the bobbin portion 52 of the lens holder 5 and fixedthereto by the use of an adhesive or the like. With this structure, thelens holder 5 is elastically supported by the suspension wires 3 and 4to be swingable and returnable to a predetermined position. In otherwords, the suspension wires 3 and 4 elastically couple the lens holder 5to the damper base 2 to make the lens holder 5 be movable in a focusingdirection which is perpendicular to the predetermined direction andparallel to an optical axis O of the objective lens 6. A combination ofthe suspension wires 3 and 4 is referred to as a suspension member.

The lens holder 5 is provided with a tracking coil, although not shownin the figure. The tracking coil serves to move the lens holder 5 in adirection perpendicular to the sheet plane of FIG. 4.

The objective lens 6 is received and held in an upper end portion of thelens holding portion 51 of the lens holder 5.

The focusing coil 7 is wound around the bobbin portion 52 of the lensholder 5 and therefore extends substantially on a plane perpendicular tothe focusing direction. In the manner which will later be described, thefocusing coil 7 serves to move the lens holder 5 in a tangentialdirection, i.e., along an optical axis O of the objective lens 6.

The first magnetic circuit 8 comprises a first yoke 81 and a firstpermanent magnet 82 and intersects one side of the focusing coil 7 whichis faced to the damper base 2. The first yoke 81 is extended between andconnected to the side walls 112 of the optical base 11. This means thatthe first magnetic circuit 8 is connected to the damper base 2 throughthe optical base 11.

The first yoke 81 has a center portion having a generally U-shapedsection. The center portion of the first yoke 81 has a first innerprotruding member 81 a and a first outer protruding member 81 bprotruding downward and located inside and outside in a radial directionof the optical pickup 1, respectively. The first inner protruding member81 a is inserted into the gap 53 of the lens holder 5. The firstpermanent magnet 82 is fixed to an inner surface of the second outerprotruding member 81 b to face the one side of the focusing coil 7 whichis faced to the damper base 2. Thus, the first magnetic circuit 8 iscoupled to the focusing coil 7 to generate first force F1 when thefocusing coil 7 is energized. The first force F1 acts to urge upwardlythe first part of the lens holder 5.

The second magnetic circuit 9 comprises a second yoke 91 and a secondpermanent magnet 92 and intersects the other side of the focusing coil 7which is opposite to the one side. The second yoke 91 is extendedbetween and connected to the side walls 112 of the optical base 11. Thismeans that the second magnetic circuit 9 is connected to the damper base2 through the optical base 11.

The second yoke 91 has a center portion having a generally U-shapedsection. The center portion of the second yoke 91 has a second innerprotruding member 91 a and a second outer protruding member 91 bprotruding downward and located inside and outside in the radialdirection of the optical pickup 1, respectively. The first innerprotruding member 81 a is inserted into the gap 53 of the lens holder 5.The second permanent magnet 92 is fixed to an inner surface of thesecond outer protruding member 91 b to face the other side of thefocusing coil 7 which is opposite to the one side. Thus, the secondmagnetic circuit 8 is coupled to the focusing coil 7 to generate secondforce F2 when the focusing coil 7 is energized. The second force F2 actsto urge upwardly the second part of the lens holder 5.

As described above, each of the first and the second magnetic circuits 8and 9 intersects with a winding of the focusing coil 7. Therefore, whenthe focusing coil 7 is supplied with an electric current, the first andthe second forces F1 and F2 are generated between the focusing coil 7and the first magnetic circuit 8 and between the focusing coil 7 and thesecond magnetic circuit 9, respectively. Under the action of the firstand the second forces F1 and F2, the lens holder 5 is moved along theoptical axis O of the objective lens 6.

Furthermore, the first and the second magnetic circuits 8 and 9 aredifferent in magnetic flux density. More particularly, the firstmagnetic circuit 8 has a first magnetic flux density while the secondmagnetic circuit 9 has a second magnetic flux density different from thefirst magnetic flux density. The first and the second forces F1 and F2are proportional to the first and the second magnetic flux densities,respectively. Herein, let a shifting force to shift the lens holder 5 berepresented by F. Then, the shifting force F is a resultant force orcombinational force of the first and the second forces F1 and F2.

It will be assumed here as a first case that the optical pickup 1 isintended to deal with the tilting variation of the optical axisillustrated in FIGS. 2A and 2B. In the first case, the first magneticflux density is determined to be higher than the second magnetic fluxdensity. The electric current is supplied to the focusing coil 7 to bekept constant. In this event, the first force F1 is greater than thesecond force F2. Thus, it is possible to prevent the tilting variationof the optical axis O illustrated in FIGS. 2A and 2B.

In a second case where the optical pickup 1 is intended to deal with thetilting variation of the optical axis illustrated in FIGS. 3A and 3B,the first magnetic flux density is determined to be lower than thesecond magnetic flux density. The electric current is supplied to thefocusing coil 7 to be kept constant. In this event, the first force F1is less than the second force F2. Thus, it is possible to prevent thetilting variation of the optical axis O illustrated in FIGS. 3A and 3B.

Therefore, the tilting variation of the optical axis of the objectivelens 6 can be avoided when the lens holder 5 is moved in the focusingdirection. As a consequence, the optical pickup 1 is prevented fromdeterioration in characteristics.

The difference between the first and the second magnetic flux densitiescan be achieved in various manners, for example, by the use of thepermanent magnets different in residual magnetic flux density, bydiffering the width of the gap between the permanent magnet and theyoke, and by allowing the leakage of magnetic flux from the yoke.

U.S. patent application Ser. No. 09/448,792, the disclosure of which isherein incorporated by reference, discloses an optical pickup in whichan objective lens is moved by the use of electromagnetic force.

What is claimed is:
 1. An optical pickup comprising: a damper base; alens holder having a first part and a second part which are spaced fromeach other in a predetermined direction; a suspension member elasticallycoupling said lens holder to said damper base to make said lens holderbe movable relative to said damper base in a focusing directionperpendicular to said predetermined direction; an objective lens held bysaid lens holder; a focusing coil fixed to said lens holder; a firstmagnetic circuit connected to said damper base and coupled to saidfocusing coil for generating first force in cooperation with saidfocusing coil to urge said first part of the lens holder in saidfocusing direction, said first magnetic circuit having a first magneticflux density; and a second magnetic circuit connected to said damperbase and coupled to said focusing coil for generating second force incooperation with said focusing coil to urge said second part of the lensholder in said focusing direction, said second magnetic circuit having asecond magnetic flux density different from said first magnetic fluxdensity.
 2. An optical pickup as claimed in claim 1, wherein said firstmagnetic flux density is determined to be higher than said secondmagnetic flux density.
 3. An optical pickup as claimed in claim 1,wherein said first magnetic flux density is determined to be lower thansaid second magnetic flux density.
 4. An optical pickup as claimed inclaim 1, wherein said focusing coil extends substantially on a planeperpendicular to said focusing direction, said first magnetic circuitintersecting said focusing coil at said first part of the lens holder,said second magnetic circuit intersecting said focusing coil at saidsecond portion of the lens holder.
 5. An optical pickup as claimed inclaim 1, wherein said suspension member is connected to said first partof the lens holder.
 6. An optical pickup as claimed in claim 1, whereinsaid suspension member comprises two pair of suspension wires which arearranged parallel to one another, each of suspension wires having an endfixed to said damper base and another end fixed to said lens holder.